Many dogs with heartworm disease show no outward signs of illness, especially early in the course of their illness. The parasites usually are diagnosed during routine blood work that is done for some other reason. The initial diagnostic protocol for dogs presenting with a deep, soft cough and signs of weakness or exercise intolerance includes a thorough history and physical examination. Most veterinarians will recommend drawing blood for a complete blood count and serum chemistry profile, and taking a urine sample for a urinalysis. These routine tests are relatively inexpensive and non-invasive and can provide a great deal of valuable information about the animal’s overall health.
Depending upon the results of the initial work-up, the veterinarian may recommend taking chest radiographs (X-rays). This is one of the best ways to assess the severity of heartworm infection, because most dogs with heavy heartworm burdens have enlarged right heart chambers and pulmonary arteries. The most accurate screening for heartworm infection is a simple blood test that detects a substance produced by adult female heartworms in infected dogs. This ELISA test (enzyme-linked immunosorbent assay and immunochromatographic test), is extremely sensitive and specific for heartworm infection. However, it is possible to get false negative test results if the infection is in its early stages, especially if only adult males are present or if there are very low numbers of adult females in the infected dog. False positive results are very uncommon.
If the antigen screening test is positive, another blood test is available to identify the concentration of microfilariae in circulation. A positive result on this test confirms that the dog is infected with adult male and female heartworms, because their offspring are in the bloodstream. If this test is negative, the animal still might be infected, because up to 25% of dogs with heartworm disease don’t have identifiable microfilariae in their blood. Again, microfilariae are the immature stage of canine heartworms.
An electrocardiogram and/or echocardiogram can also be used to assess moderate to severe cases. Adult heartworms can be seen in the vena cava of dogs with caval syndrome using these more advanced diagnostic protocols.